When ht4u.net reviewed the Sapphire Radeon HD 7870 XT they measured the VRM temperature and it was 95°C. The power MOSFETs, used in the VRM circuit of that card, have a maximum temperature rating of 125°C.
The VTX3D Radeon HD 7870 Black Edition, which is also a Boost Edition card, had a measured VRM temperature of 94.1°C.